The present invention generally relates to misfire detection in internal combustion engines and, more particularly, to a method and system which employ an indicated mean effective pressure (IMEP) parameter for misfire monitoring of an internal combustion engine.
In the motor vehicle industry, a large effort is being devoted to the development of continuous, on-board monitoring of systems and components affecting vehicle tailpipe emissions. A particularly vexing situation is the diagnosis by On Board Diagnostics (OBD) of engine combustion failures, known as misfire detection. This task must be performed under virtually all operating conditions. Misfires can arise following malfunctioning of an injector, loss of spark from a spark plug, mixture too lean or to rich, and the like.
The misfire identification rates must be low as one percent and identify the event quickly so as to prevent deterioration of the emissions control system. Such a diagnostic method must operate continuously, in real time, on all vehicles, monitoring every engine cylinder combustion event. False alarm immunity is an important concern, because the consequence of exceeding the misfire limit is illumination of the malfunction light and a trip made by the driver to a repair facility. On the other hand, high misfire detection efficiency and identification accuracy is necessary to determine whether the vehicle emissions are proper.
Direct injection gasolines engines are currently being developed to increase fuel economy and thus reduce emissions. One of the advantages of direct engine architecture is the ability to run stratified combustion charges. By moving the fuel injector into the combustion chamber the fuel can be positioned precisely in the combustion chamber. This enables positioning a rich pocket near the spark plug while having a leaner charge elsewhere in the combustion chamber. Stratified combustion charges allow dimensions (or variables) to be optimized to increase fuel economy during engine operation. The ability to control air/fuel ratios (10:1 through 60:1) and combustion types (homogenous, homogenous lean, and stratified lean) can increase the fuel economy through the ability to operate in the most efficient manner at each engine operating point.
Current spark ignition engines run in the homogenous stoichiometric mode. Traditionally, this means controlling the air/fuel ratio to around 14.6:1 throughout the engine operating range. The engine load is one of the parameters used to define the engine torque output. The engine load is generally defined as a ratio of the current cylinder air charge divided by the potential maximum cylinder air charge. For an engine inducting one half of the potential maximum amount of cylinder charge, the engine load would be 0.5 or 50%. Because this parameter is a ratio, it is normalized for engine displacement. This allows a consistent value irrespective of engine displacement and calibrations from various engine displacements can be compared quickly.
The current trend from homogenous to stratified charge operation renders the engine load parameter useless. The controlling factor for engine output (or torque) is no longer the amount of air drawn into the cylinder. The controlling factor is now the amount of fuel consumed. In more convenient terms, the controlling factor for engine output is the air/fuel ratio and the combustion mode. This leads engine control system designers to develop engine torque based algorithms. The amount of engine torque desired by the driver is converted to an air/fuel ratio and an engine operating mode. These parameters (and others) can then be optimized to obtain the best fuel economy in consideration with driveability, emissions, and other factors.
New engine control algorithms provide a challenge to the misfire monitor OBD algorithm. Engine torque is currently used by the OBD algorithm to detect engine combustion misfires by comparing back to the calculated engine deceleration parameter. Current OBD algorithms use an engine speed versus engine load table to provide the output threshold to detect a misfire.
As demonstrated above, it is not possible to apply the current OBD algorithm normalization parameters. A more useful means for normalizing the detection threshold criteria across the operating region is needed.
It is an object of the present invention to provide a method and system which employ an indicated mean effective pressure (IMEP) parameter for misfire monitoring of an internal combustion engine.
Accordingly, the present invention provides a method for detecting misfire in an internal combustion engine of a vehicle. The method includes determining displacement and torque of the engine. An indicated mean effective pressure (IMEP) parameter is then calculated for the engine. The indicated mean effective pressure (IMEP) parameter is engine torque divided by engine displacement. A misfire monitor monitors the indicated mean effective pressure (IMEP) parameter to detect misfire.
Preferably, the method further includes determining maximum indicated mean effective pressure (IMEP_MAX) parameter for the engine. A normalized indicated mean effective pressure (IMEP_LOAD) parameter for the engine is then calculated. The normalized indicated mean effective pressure (IMEP_LOAD) parameter is the indicated mean effective pressure (IMEP) parameter divided by the maximum indicated mean effective pressure (IMEP_MAX) parameter. Monitoring the indicated mean effective pressure (IMEP) parameter to detect misfire includes monitoring the normalized indicated mean effective pressure (IMEP_LOAD) parameter to detect misfire.
Further according to the present invention, there is provided a system for detecting misfire in an internal combustion engine of a vehicle. The system includes means for determining displacement of the engine and means for determining torque of the engine. A processor calculates an indicated mean effective pressure (IMEP) parameter for the engine. The indicated mean effective pressure (IMEP) parameter is engine torque divided by engine displacement. A misfire monitor monitors the indicated mean effective pressure (IMEP) parameter to detect misfire.